Aircraft navigation is becoming increasingly automated with attendant improved accuracies. In aviation not only is it typically important to determine ones position on the map, but it is also important to determine ones height above the geographical features represented on the map. This vertical height determination can be more important during take off and landing. During take off and landing the aircraft comes closer to the ground, where variations in height determinations impact the pilot's ability to land. Aside from basic accuracy of an avionics system under good conditions rough terrain, ocean approaches, and other problematic approach conditions can often present obstacles to determining height so that a successful landing may be provided. The problem of height determination may be made more acute in adverse weather conditions where visibility can be reduced.
Avionics have progressed from visually verifying ones position on a map based on hand calculations and determining height based on an altimeter reading. Some early systems at airports provided radar beams, or other radio signals (typically produced at the air field) to guide aircraft in for a landing. An example of such a system is the Instrument Landing System (ILS). Modern avionics designers may also wish make use of satellite navigation systems, such as Global Positioning System (“GPS”), to determine aircraft position. However, GPS is typically not sufficiently accurate, especially in determining vertical distances.
GPS-only solutions may be used in a Non-Precision Approach (“NPA”) to descend to not less than 250 feet Height Above Touch Down Zone (“HATDZ”). GPS with a Space-Based Augmentation System (“SBAS”) may be used in a Precision Approach to descend to not less than 200 feet HATDZ which is equivalent to using a Cat I ILS ground installation at the airport. GPS with a Ground-Based Augmentation System (“GBAS”) may be used in a Precision Approach to descend to not less than 100 feet HATDZ which is equivalent to using a Cat II ILS installation. There are no GPS-based solutions that provide equivalent performance to a CAT III ILS installation.
Runway information is available in a standard database ARINC 424 format that provides the locations of the runway endpoints (i.e., the thresholds); however, glide path angles and HATDZ measurements typically must be made from the touchdown zone. Since it is very rare for a runway to have a significant slope (change in elevation) from end-to-end, the elevation of the touchdown zone is the same as the threshold. Thus, for consistency, all references to aircraft altitude above runway are typically HATDZ.
Examples of SBAS solutions include the Wide Area Augmentation System (“WAAS”) in the United States. Other exemplary implementations include the Indian Gagan, the European Geostationary Navigation Overlay Service (EGNOS), the Japanese Multi-functional Satellite Augmentation System (MSAS), the StarFire system, the OmniSTAR system, and the like. These SBAS implementations will allow an aircraft to descend to 200 ft HATDZ (Cat I minima) at airports that do not have ILS installations. An example of a GBAS implementation is the Local Area Augmentation System (LAAS) in the United States which will support descent to 100 ft HATDZ (Cat II minima) at airports that do not have ILS installations. However, the cost of a LAAS installation is roughly equivalent to the cost of the ILS installation that it is intended to replace. So LAAS is not widely available. Therefore, it is desirable to provide an autonomous solution (i.e., carried on the aircraft and not dependent on ground installations) to allow descent below 200 ft HATDZ to Cat II or even Cat III minima.